Various types of vehicle height adjusting systems are known. Hydraulic systems and hydro-pneumatic systems are most commonly used for vehicle height adjusting systems. To the end of improving the control precision and simplifying the structure, a proposal has been made to use a worm gear mechanism or a ball screw mechanism powered by an electric motor (see Japanese patent laid open publication No. 11-108100). In this prior proposal, between an upper spring seat supporting an upper, end of a suspension spring and a vehicle body member is interposed a ball screw actuator mechanism. A rotor (male thread member) of the ball screw actuator mechanism is driven by an electric motor so that the space between the upper spring seat and vehicle body member, and hence the vehicle height may be adjusted. In this vehicle height adjusting system, to prevent the electric motor from being turned by the load and to minimize the size of the electric motor, a worm reduction gear mechanism is interposed between the electric motor and rotor.
However, the use of a worm gear mechanism having a relatively low mechanical efficiency gives rise to a number of problems. Because a worm gear mechanism causes a significant torque loss, the electric motor is required to have a correspondingly large output, and this prevents a compact and economical design. A large power consumption is also a problem. These problems may be alleviated to a certain extent by combining the use of a ball screw having a high mechanical efficiency as proposed in the aforementioned Japanese patent publication. However, the use of a ball screw mechanism not only increases the manufacturing cost but also creates the need to prevent the rotation of the female thread member to prevent unintended movement of the ball screw mechanism when an input from the road is applied thereto. However, no such measure is described in this prior application.
Japanese patent publication (kokoku) No. 08-019971 discloses an actuator for a clutch that uses a pair of cylindrical members in a mutually threading engagement in a coaxial relationship to actuate the clutch with an axial relative displacement between them. The two cylindrical members are provided with spur gears that are driven at slightly different gear ratios by corresponding pinion gears mounted on a common drive shaft. The resulting differential rotation between the two cylindrical members is transformed into a relative linear movement thereof by the threading engagement.
This provides a highly compact and high gear ratio mechanism, but the spur gear mounted on the outer cylindrical member is splined thereto to accommodate the linear (axial) movement of the outer cylindrical member in relation to the axially fixed pinion gear and at a position significantly remote from the point of the threading engagement. Therefore, the resulting moment causes a tilting of the outer cylindrical member and this inevitably applies uneven loads to the threading engagement as well as to the spline engagement. Therefore, some improvement is required for this mechanism to be used in a vehicle height adjusting mechanism. Also, the mechanism that can be used in a vehicle height adjusting mechanism must be capable of withstanding the impulsive and oscillatory loads that are applied to the load bearing end of the mechanism.
In such a vehicle height adjusting system, it is important that it is irreversible in the sense that the vibrations, impacts and other inputs from the road surface would not affect the height of the vehicle. It should be ensured that the vehicle height would not change even if there are changes in the viscosity of the lubricating oil or in the friction in the gear meshing parts over time unless it is intended.